1. Field of the Invention
This invention relates to a projection lens, and in particular to a projection lens for video projector for projecting the image of a CRT (cathode ray tube) and obtaining a large picture plane.
2. Description of the Prior Art
Generally, in a video projector, CRTs of three colors, i.e., B (blue), G (green) and R (red), are projected onto a screen by respective lenses and all of the light emission characteristics of the three colors are narrow in spectrum width and therefore, there is little need for an achromatic lens. As the lenses for video projector, there are known ones comprising only ordinary spherical lenses and ones including non-spherical surfaces. In the lenses comprising only spherical surfaces, there is a limitation in endowing it with a bright and wide angle of view, and it is indispensable to use non-spherical surfaces in order to reduce the number of component lenses and maintain a high performance. As the projection lenses of this type using non-spherical surfaces, there are known ones disclosed in publications such as Japanese Laid-open Patent Application No. 124114/1980 (corresponding U.S. Pat. No. 4,300,817) and Japanese Laid-open Patent Application No. 108818/1982. These projection lenses each comprise three lenses and are relatively simple in construction, but their imaging performance is still insufficient. Particularly, projection lenses for video projector having a high imaging performance which may be available for so-called high-quality televisions of 1125 scanning lines recently developed or high accuracy image televisions which are being developed as described, for example, in OPTICAL AND ELECTRO-OPTICAL ENGINEERING CONTACT (Vol. 20, No. 8, pp. 59-61) have been desired.
The performance of the conventional projection lens for video projector will hereinafter be described with respect to its MTF (modulation transfer function) characteristic. FIG. 1 of the accompanying drawings shows the construction of a lens disclosed in Japanese Laid-open Patent Application No. 124114/1980 (corresponding U.S. Pat. No. 4,300,817), and FIG. 2 of the accompanying drawings shows the CRT side MTF of a first embodiment thereof and has been submitted by the applicant for evaluation of the performance in the stage of examination. With regard to 5 lines/mm on the CRT side, an MTF value of 50% is obtained at the center, but with regard to the angles of view of 100% (H=-833 mm), 85% (H=-708 mm), 70% (H=-583 mm) and 35% (H=-291 mm), MTF values of the order of 40% are only obtained. FIG. 3 of the accompanying drawings shows the lens construction of a lens disclosed as a first embodiment in Japanese Laid-open Patent Application No. 108818/1982, and FIG. 4 of the accompanying drawings shows the screen side MTF thereof. In this example, the periphery of the screen is curved with a radius of curvature of 2500 mm so as to be approximate to a lens. This screen side MTF value has been calculated with respect to 0.5 line/mm, but even if, for comparison with the former, it is multiplied by a projection magnification 9.78 to obtain a value of 4.89 lines/mm on the CRT side, this will be equivalent. Even for this MTF value, a value of 50% is obtained except for the meridional direction components (indicated by dotted lines in the Figure) of the angles of view of 85% and 70%, and this is a better value than the former but cannot yet be said to have a sufficient imaging performance.